1Department of Biomedical Engineering, The University of Virginia, Charlottesville, VA 22908, USA2Department of Physics, The University of Virginia, Charlottesville, Virginia 22908, USA3Department of Radiology and Medical Imaging, The University of VA, Charlottesville, VA 22908, USA4Department of Pediatrics, The University of VA, Charlottesville, Virginia 22908, USA

Received 17 June 2011; Revised 14 September 2011; Accepted 17 September 2011